Medium and high voltage switching installations, for voltages of over 1000 V, are widely used with SF6 insulation due to its good insulation properties, allowing to obtain compact installations. There is however a strong wish to move away from SF6 as insulation medium, on account of the negative consequences of SF6 for the environment—notably of its decomposition products HF, SOF2 and SO2—. Therefore, air is used more and more as an insulation medium. As a consequence of the factor 2 to 4 less well insulation quality of air, the size of the installation needs to be increased to prevent electrical sparkover or arcing between the various voltage-carrying parts. This in turn leads to an increase in the amount of energy released upon occasional arcing, as the amount of energy is proportional to the size of the arc, which is proportional to the distance between the conductive parts.
Without further measures, the energetic arcing will continue until the voltage is taken of the voltage-carrying parts, which is generally done by switching the installation off of the grid with an external switch further up in the grid in a power supply line feeding the switching installation. Such external switching is generally done after about one second after the misbehaviour of the switching installation has been detected by an external monitoring system in the grid. One second of energetic arcing may however result in a significant danger and damage. The switching installation thus has to be equipped with a measure to take away the energetic arc that is sufficiently fast to reduce the effects to acceptable levels.
A switching installation with an air-filled enclosure with arc protection is described in EP 1 463 172 A1, in which the input conductor is automatically grounded upon the occurrence of an energetic arc in the enclosure, in order to stop the arcing. It relies on a pressure increase in the enclosure upon the occurrence of an energetic arc and the resulting heating of the air. The enclosure comprises a venting panel, which opens upon an increase of the pressure in the enclosure and, via a flexible cable, acts on an earthing switch. The earthing switch short-circuits the conductor to ground, such that the arcing stops.
The known switching installation has the drawback that, although the arcing is stopped, hot gasses are released through the venting panel and may thus comprise a safety risk for operating personnel in the vicinity of the switching installation. It is further believed that the mechanism used in EP 1 463 172 A1 for grounding the input conductor is not always sufficiently reliable.
Another example of a switching installation in which pressure generated by an internal arc is used for actuating a grounding switch is shown in EP 0 235 755 A1. Also in this installation, actuation of the grounding switch causes the internal arc to dissappear. It is further believed that the robustness, and hence the safety, of the mechanism used in EP 0 235 755 A1 for actuating the grounding switch is not optimal.
A robust arc protection mechanism is also beneficial for systems using SF6 insulation. Although an arc in such a system is shorter, hence less energetic, arcing may still cause damage to the switching installation, which may cause a unwanted release of hot gasses, such as the SF6-decomposition products, and more important a safety risk for operating personnel. Moreover, as SF6 is a greenhouse gas, release of SF6 is unwanted for environmental reasons.